Unfortunately accumulators are bulky, heavy, and expensive, and nearly half the energy of the original charging current is lost in obtaining the reversed or working current. They are therefore not as yet adapted for general use, though perfectly capable of supplying either light or motive power, for both which purposes they have been successfully applied in special cases. The future both of electric power and electric lighting is now reduced entirely to a question of cost; and though it is hard to beat gas and the steam-engine, with cheap coal, and air and water for nothing, it is possible that by using natural sources of power to move dynamos, and by obtaining zinc back as a residual product in batteries, electricity may in certain cases carry the day. CHAPTER V. POLARITY IN MATTER. Ultimate elements of universe—Built up by polarity—Experiment with magnet—Chemical affinity—Atomic poles—Alkalies and acids—Quantivalence—Atomicity—Isomerism—Chemical stability—Thermochemistry—Definition of atoms—All matter built up by polar forces. I almost fear that by this time some of my readers may think that I have seduced them under false pretences to read long chapters of dry science, when they had been led from the introduction to anticipate discussions on the more immediately interesting topics of morals, religions, and philosophies. My excuse must be that these scientific subjects are really of extreme interest in themselves and indispensable as a solid basis for the superstructure to be raised on them. How can I attempt to show that the law of polarity extends to the more complex problems of human thought and life, if I fail in establishing its application to the simpler case of inorganic force and matter? It must be recollected also that among the primitive polarities is that of author and reader. It is my part to endeavour to present the leading facts and laws of the material universe in such plain and popular language that the ordinary reader who has neither time nor faculty for special studies may apprehend them clearly without excessive effort, or extraordinary intelligence. But it is the reader’s part to supply a fair average amount of attention, and above all to feel an interest in interesting matters. Cleverness and curiosity are very much convertible terms, and the clearest exposition is thrown away on the torpid mind which views the marvellous universe in which he has the privilege to live, with the stupid apathy of the savage, taking things as they come without caring to know anything about them. For the reader’s part of the work I am not responsible; but for my own I am, and I proceed therefore to give in my own way, and with the best faculty that is in me, a clear summary of such of the fundamental facts and laws of nature as seem necessary for the work I have undertaken.